arch-12: Expand hex registry to full Space2Stats catalog

[koito19960406]

Overview

inst/hex_vars_registry.yaml v0.1.0 ships with one real indicator (flood_exposure_15cm_1in100) and the internal population variable. A deployer has no meaningful choice of hex-derived indicators today.

The WB Space2Stats Database (open access, MIT-licensed) publishes pre-computed H3 Level-6 summary statistics globally across 6 thematic collections (~108 columns). This tracker covers expanding the registry to the full catalog so deployers can compose country PTIs from the Space2Stats atlas without bringing their own data.

Design doc: .github/docs/arch-12-hex-catalog-expansion.md

Collections to integrate

Collection	Theme	Backend
space2stats_population_2020	Demographics	parquet
urbanization_ghssmod	Urbanization	parquet
nighttime_lights	Economic activity (VIIRS NTL 2012–2024)	parquet
flood_exposure_15cm_1in100	Climate hazards	parquet ✓ done
builtarea_ghsl	Infrastructure (GHSL 1975–2030)	parquet
climate	Climate hazards + SPI time series	parquet + REST

Sub-issues

• A — Discover & document Space2Stats parquet asset URLs (research, no code)
• B — Add space2stats_population_2020 source (YAML-only)
• C — Add urbanization_ghssmod source (YAML-only)
• D — Add nighttime_lights source (YAML-only)
• E — Add builtarea_ghsl source (YAML-only)
• F — Add REST backend (backend: "rest") + climate static columns (code PR)
• G — Add climate time-series via REST (YAML-only, depends on F)

Definition of Done

• list_hex_vars() returns ≥ 100 variables (vs. 2 today).
• Rwanda (rwa_shp, 507 H3-6 cells) pipeline run fetching one variable from each new collection completes without warnings.
• R CMD check stays 0/0/0.

Depends on

arch-11 hex data access pipeline (fully landed on main).